In a cellular telecommunications network, the geographical area covered by the network is divided into cells with groups of one or more cells defining a service area of the network. Each of the cells contains a base station that transmits and receives control information and voice communications to and from mobile stations that are located in the coverage area of the cell. Typically, each of the cells uses a separate control channel for exchanging control information with a mobile station, and a set of voice channels for exchanging voice or data signals (traffic signals) with a mobile station. The type of control channel used in a particular system depends on the system standard. The most popular current cellular networks employ either an analog control channel (ACCH) or a digital control channel (DCCH), or both an analog control channel (ACCH) and a digital control channel (DCCH).
The term ACCH was created to describe a control channel originally developed for use in a system having analog voice channels, although now an ACCH may also be used in a dual mode analog/digital system for control of digital traffic channels. Each ACCH of a system is typically assigned to a set frequency channel. An example of a network using an ACCH would be a cellular network that operates according to the Telecommunications Industry Association/Electronic Industrial Association (TIA/EIA) 553 analog system standard (AMPS). In AMPS, approximately 21 of the 832 available frequency channels are defined for use as ACCHs.
The term DCCH is commonly used to describe a control channel used in a time division multiple access (TDMA) digital cellular system. In a TDMA system, each channel which is broadcasting at a particular frequency is divided into a plurality of time slots. Subscriber communications or control channel signals are converted to digital format and divided into short communication bursts. Each burst is tagged with an identifier, assigned a time slot, and is broadcast in an interleaved fashion with other bursts on the same frequency. At the receiving end, the identifiers are used to reconstruct the entire communication from the individual bursts. An example of a system using a DCCH would be a cellular network operating according to the TIA/EIA IS-136 dual mode standard.
In a cellular system, a control channel must always be available for each mobile station operating within the system. This is necessary so the mobile station always has access to the control information needed to make and receive calls and otherwise to operate correctly in the system. When a mobile station is powered on at an arbitrary location within a cell of the system, the mobile station must initially locate and select a suitable control channel. As the mobile station then roams about the coverage area of the cellular system, it must be able to locate and tune to a control channel of the cells into which it roams. Control channel selection while roaming is typically performed by making measurements on control channels identified in a "neighbor cell list". The neighbor cell list, which identifies the control channels of neighboring cells, is transmitted on the control channel of each cell at certain predetermined intervals. As the mobile station roams, it measures each neighbor cell control channel in the list at predetermined times and tunes to one of the neighbor cell list control channels when the signal strength of the signal received on that control channel becomes greater than the signal strength received from the current control channel. Since the configuration of the cellular system is known, determining the correct control channels for a particular cell's neighbor cell list is a relatively straightforward task.
If a mobile station does not have or has limited or outdated information on the identity of the nearest control channels, the process of locating a suitable control channel may present problems that do not occur when a mobile station has accurate identifying information on available or known neighbor cell control channels. For example, power-up of a mobile station may take place arbitrarily within any cell of the system, so initial control channel selection can also arbitrarily take place within any cell of the system. In the case of initial control channel selection, the mobile station must be capable of selecting a suitable control channel that could be one of any of the whole set of control channels in the system.
The use of DCCHs in a cellular system presents some problems in the control channel selection process that do not commonly occur when only ACCHs are used and when a DCCH is preferred for use in the cellular system. Since an ACCH is typically assigned to a fixed frequency in a cellular network, the frequency channels can be known at the mobile station. In an AMPS network, for example, at least one of these 21 frequency channels assigned as control channels in the AMPS standard is used in each base station of the network. It is a relatively simple task for the mobile station to scan and measure the 21 frequencies at certain times, during power-up for example, to determine the control channel with the strongest received signal strength at the mobile station and tune to that frequency channel. That control channel would normally be the ACCH of the cell in which the mobile station is located. The mobile station does not need to know the control channel of the cell in which it was located or the neighbor cell list of that cell before power-up in order to quickly locate a control channel. DCCHs on the other hand are configured as a certain time slot or set of time slots on a frequency channel in a TDMA system, and for flexibility it is common to allow assignment of a DCCH to any one of a large number of frequency channels, possibly the whole range of frequency channels available to the base stations in the network. For example, in an IS-136 system, the DCCHs assigned to the various base stations could be on any network frequency channel in the first and fourth time slots. During control channel selection, a mobile station may have to scan the whole range of frequencies in order to locate and select a DCCH. This process could take a considerable period of time.
Different techniques have been developed for DCCH selection in situations such as power-up. Several solutions are included in the reference model process described in the IS-136 standard (IS-136.1-A, Appendix D). In this process, a mobile station initially tunes to the last-used DCCH or tunes to a strongest DCCH chosen from a number of last-used DCCHs. The mobile then synchronizes to the DCCH and determines if the signal strength and service criteria are satisfied on that DCCH. If the criteria are satisfied, the mobile station then camps on the DCCH. When camped on the DCCH, the mobile station monitors that DCCH for messages from the system and also can initiate calls on the DCCH. If the signal strength and service criteria are not satisfied, the mobile station repeats the process on the next strongest last-used DCCH if more than one DCCH identifier is stored. If an acceptable last-used DCCH cannot be found, the mobile station initiates a scan of the ACCHs of the system and tunes to the best acceptable ACCH that is found. If an ACCH is found, the mobile station monitors the ACCH and may receive a DCCH pointer that identifies a DCCH in the control channel information message of the ACCH. If a DCCH pointer is received, the mobile station performs the IS-136 control channel selection procedure on the DCCH identified by the pointer. If no DCCH pointer is received on the ACCH or if the DCCH pointed to does not meet the control channel selection procedure criteria, the mobile station registers on the ACCH. If the mobile station is unable to find an ACCH, it performs a band scan of the entire system band. The mobile station then finds the strongest RF channel in the band. If the strongest RF channel includes a DCCH, the mobile station performs the IS-136 control channel selection procedure and camps on the DCCH if the selection criteria are satisfied. If a DCCH is found but the selection criteria are not satisfied on the RF channel having the strongest received signal strength (rssi) at the mobile station, the mobile station receives a neighbor cell list from the DCCH and attempts to locate an acceptable neighbor DCCH for camping. If a DCCH is not found on the current RF channel, the mobile station will look for a digital traffic channel (DTC) on the current RF channel carrying coded digital control channel locator (CDL) information. If CDL information is found, the mobile station attempts to find a DCCH that meets selection criteria from the CDL information, starting with the strongest DCCH. If need be, the mobile station will check all DCCHs identified in the CDL. If no DCCH is located from the strongest RF channel, the mobile station selects the next strongest RF channel and repeats the process for each RF channel, continuing until all channels of suitable signal strength have been evaluated. If no DCCH is found from the RF channel search, the mobile station will perform a search on an alternate system band or start the process again.
A mobile station may transition, or perform unnecessary access attempts, transitioning back and forth between the idle state on an ACCH and camping on a DCCH multiple times when operating in an IS-136 system. This may occur in the above situation, for example, when the DCCH pointed to by an ACCH pointer does not meet DCCH selection criteria and the mobile station registers back on the ACCH after failing to camp on the DCCH. In this situation, a received signal strength threshold, originally set to the predefined received signal strength (rssi) threshold, is set to a value defined as RSS_ACC_MIN and held at that level, so that access is not attempted to a DCCH from the ACCH unless the measured rssi of the DCCH meets the threshold RSS_ACC_MIN. In this situation, if the rssi of a DCCH has a value near the threshold RSS_ACC_MIN, the mobile station may attempt to access the DCCH and fail to camp on the DCCH, or fall back to an ACCH soon after camping on the DCCH due to weak signal strength on the DCCH. This could be repeated over and over.
If the mobile station frequently and unnecessarily moves back and forth between an ACCH and a DCCH, the mobile station may miss incoming calls, and power consumption will be higher than if the mobile station stays camped on a DCCH or idle on an ACCH.